1. Field of the Invention
This invention relates to voltage regulation in a power inverter, and more particularly to voltage regulation using pulse width modulation (PWM).
2. Description of the Prior Art
The use of PWM to provide voltage regulation and harmonic reduction in the fundamental sine wave provided by an inverter is well known in the art. Briefly stated, known PWM techniques involve chopping the fundamental frequency (f.sub.0) rectangular waveform from the inverter bridge with one or more notches in the positive and negative portions in each cycle of the fundamental signal. These notches are fixed in a pattern which positions them at selected electrical degrees (360.degree..multidot.f.sub.0 .multidot.T) from the positive and negative going transition of the rectangular wave, the notch pattern of the positive portion of the wave representing a mirror image of the notch pattern in the negative portion. The notch pulse widths are similarly defined in electrical degrees and the number of notches, their location and pulse width determine the amplitude of the fundamental sine wave signal and its harmonics, as discussed in detail in U.S. Pat. No. 3,324,376 to J. M. Hunt.
The PWM notch patterns are grouped into the two general categories of edge notch patterns, where the notches are located within 60 degrees from the zero and 180 degree positions of the rectangular waveform period, and center notch patterns which locate the notches plus or minus 30 degrees symmetrically about the 90 and 270 degree positions of the fundamental waveform. Edge notch and center notch PWM waveforms have distinctly different effects on the fundamental signal and harmonic signal amplitudes. The fundamental signal amplitude is far less sensitive to edge notches than to equal number center notches of equal pulse width, whereas the reverse is true of the harmonic signal amplitudes. Depending on the inverter application, i.e. in motor applications the harmonic amplitudes may be of greater concern than the amplitude of the fundamental driving signal, whereas in a precision instrumentation system the voltage amplitude and RMS value of the fundamental may be of far greater concern than the harmonic content, an obvious approach is to select either edge notching or center notching with one or more patterns to provide for a variation in notch location and notch width.
Since edge notch PWM requires pulse widths on the order of twice that required by center notching to produce the same reduction in the fundamental signal voltage amplitude, edge notching provides for less reduction in voltage amplitude which is desirable for heavily loaded inverters where the maximum output voltage is desired. The more coarse center notching has application under different conditions, in inverters having light loads and a high DC input voltage where small variations in the load may result in large amplitude variations of the fundamental, the center notching allowing for restoration of the regulated amplitude within the shorter response time. In some prior art systems, such as power inverters having a wide range of current loads, a combination of both edge notching and center notching are used; however, this requires discrete switching between edge notch and center notch patterns in response to the amplitude changes. This discrete switching results in voltage transients at the output of the inverter which increase the harmonic content of the fundamental and which may result in damage to the inverter load, all of which is undesirable in a power inverter such as that used by a utility providing commercial power.